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LECTURE 1
Introduction:
The term "soil" can have different meanings, depending upon the field in which it is considered.
To a geologist, it is the material in the relative thin zone of the Earth's surface within which roots occur, and which
are formed as the products of past surface processes. The rest of the crust is grouped under the term "rock".
To a pedologist, it is the substance existing on the surface, which supports plant life.
To an engineer, it is a material that can be:
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built on: foundations of buildings, bridges
built in: basements, culverts, tunnels
built with: embankments, roads, dams
supported: retaining walls
Soil Mechanics is a discipline of Civil Engineering involving the study of soil, its behaviour and application as an
engineering material.
Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with
sediments and other unconsolidated accumulations of solid particles, which are produced by the mechanical and
chemical disintegration of rocks, regardless of whether or not they contain an admixture of organic constituents.
Soil consists of a multiphase aggregation of solid particles, water, and air. This fundamental composition gives rise
to unique engineering properties, and the description of its mechanical behavior requires some of the most classic
principles of engineering mechanics.
Engineers are concerned with soil's mechanical properties: permeability, stiffness, and strength. These depend
primarily on the nature of the soil grains, the current stress, the water content and unit weight.
Formation of Soils:
Soil is formed from rock due to erosion and weathering action. Igneous rock is the basic rock formed from the
crystallization of molten magma. This rock is formed either inside the earth or on the surface. These rocks
undergo metamorphism under high temperature and pressure to form Metamorphic rocks. Both Igneous and
metamorphic
rocks are converted
in to sedimentary
rocks due to transportation to different locations
by the agencies such as wind, water etc. Finally, near the surface millions of years of erosion and weathering
converts rocks in to soil.
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Fig. Geologic Cycle of Soil
Soils are formed from materials that have resulted from the disintegration of rocks by various processes of physical
and chemical weathering. The nature and structure of a given soil depends on the processes and conditions that
formed it:
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Breakdown of parent rock: weathering, decomposition, erosion.
Transportation to site of final deposition: gravity, flowing water, ice, wind.
Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice or
overlying deposits, change from saline to freshwater, leaching, contamination.
All soils originate, directly or indirectly, from different rock types.
Weathering:
Physical weathering reduces the size of the parent rock material, without any change in the original composition of
the parent rock. Physical or mechanical processes taking place on the earth's surface include the actions of water,
frost, temperature changes, wind and ice. They cause disintegration and the products are mainly coarse soils.
The main processes involved are exfoliation, unloading, erosion, freezing, and thawing. The principal cause is
climatic change. In exfoliation, the outer shell separates from the main rock. Heavy rain and wind cause erosion of
the rock surface. Adverse temperature changes produce fragments due to different thermal coefficients of rock
minerals. The effect is more for freeze-thaw cycles.
Chemical weathering not only breaks up the material into smaller particles but alters the nature of the original parent
rock itself. The main processes responsible are hydration, oxidation, and carbonation. New compounds are formed
due to the chemical alterations.
Rain water that comes in contact with the rock surface reacts to form hydrated oxides, carbonates and sulphates. If
there is a volume increase, the disintegration continues. Due to leaching, water-soluble materials are washed away
and rocks lose their cementing properties.
Chemical weathering occurs in wet and warm conditions and consists of degradation by decomposition and/or
alteration. The results of chemical weathering are generally fine soils with altered mineral grains.
The effects of weathering and transportation mainly determine the basic nature of the soil (size, shape, composition
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